Mechanism of formation of alkali metal cation adducts in plasma desorption mass spectrometry of biomolecules

Experimental data on correlations between the directions of the incident primary ion and the ejected protonated M + H]⁺ and alkali metal adduct M + Cs]⁺ molecules for three peptide samples with an incident beam of 72.3 MeV ¹²⁷I¹⁴⁺ ions are reported. Measurements were carried out in a linear time-of-flight mass spectrometer by monitoring the secondary ion yield as a function of electrostatic deflection in a direction perpendicular to the spectrometer axis. No difference was observed in the direction in which M + H]⁺ and M + Cs]⁺ ionic species are preferentially desorbed. The results obtained suggest that alkali metal cation attachment to biomolecules in plasma desorption mass spectrometry is realized in a close spatial location and time interval with protonation. Formation of ion-molecule complexes occurs at an early stage of the desorption event and precedes their ejection into the gas phase.